1. Field of the Invention
The present invention relates to OFDM (orthogonal frequency division multiplexing) demodulators and receivers for demodulating OFDM signals and methods for the same.
2. Description of the Related Art
A modulation system called orthogonal frequency division multiplexing (OFDM) system (hereinafter, simply referred to as an “OFDM system”) is used for transmitting digital signals. In the OFDM system, a transmission band is provided with a number of orthogonal subcarriers, and data are assigned to the amplitude and the phase of each subcarrier with PSK (phase shift keying) and QAM (quadrature amplitude modulation) to perform digital modulation.
Application of the OFDM system to digital terrestrial broadcasting that is strongly affected by multipath interference has been widely discussed. Standards for digital terrestrial broadcasting employing the OFDM system include, for example, DVB-T (digital video broadcasting-terrestrial), ISDB-T (integrated services digital broadcasting-terrestrial), and ISDB-TSB (ISBD-T sound broadcasting).
As shown in FIG. 5, transmission signals according to the OFDM system are transmitted on a symbol-by-symbol basis. The symbols are called OFDM symbols. An OFDM symbol is constituted by an effective symbol representing a signal period during which an IFFT (inverse fast Fourier transform) operation is performed for transmission and a guard interval where a rear part of a waveform of the effective symbol is copied. The guard interval is arranged in a front part of the OFDM symbol. The duration of the guard interval is set at a quarter or one-eighth of the duration of the effective symbol.
OFDM receivers for receiving such OFDM signals perform an FFT (fast Fourier transform) operation using a FFT processing circuit to demodulate received OFDM signals. The OFDM receivers detect boundary positions of OFDM symbols, each of which is constituted by an effective symbol and a guard interval. The OFDM receivers then set a scope of the FFT operation (FFT window) having the same length as the effective symbol on the basis of the detected symbol boundary positions. The OFDM receivers extract data included in the part set as the FFT window from the OFDM symbol, and perform the FFT operation on the data.
In addition, the OFDM system specifies that a plurality of the above-described OFDM symbols collectively constitutes a unit of transmission called an OFDM frame. For example, in the ISDB-T standard, 204 OFDM symbols constitute an OFDM frame. In the OFDM system, for example, insertion positions of pilot signals are set for each OFDM frame as a unit.
In an OFDM system employing QAM system for modulating data signals onto each subcarrier, distortion is caused in the signals modulated onto each subcarrier by influences of a multipath or the like during transmission. This undesirably alters the characteristics of the amplitude and phase of each subcarrier. Accordingly, a receiving side is required to equalize the waveform of the received signals so that the amplitude and phase of each subcarrier are equalized. In the OFDM system, a transmitting side scatters pilot signals having a specific amplitude and a specific phase over OFDM symbols of transmission signals. The receiving side monitors the amplitude and phase of these pilot signals so as to determine the frequency characteristics of a transmission path. The receiving side is configured to equalize the received signals using the determined transmission path characteristics. In the OFDM system, pilot signals used for calculation of transmission path characteristics are called scattered pilot signals (hereinafter, referred to as “SP signals”).
FIG. 6 shows an arrangement pattern of SP signals in OFDM symbols according to the DVB-T standard and the ISBD-T standard.
In the DVB-T standard and the ISBD-T standard, a BPSK (Binary Phase Shift Keying)-modulated SP signal is inserted every twelve subcarriers in a subcarrier dimension (frequency dimension). Additionally, in the DVB-T standard and the ISBD-T standard, an insertion position of an SP signal is shifted in the frequency dimension by three subcarriers for each OFDM symbol. As a result, an SP signal is inserted every four OFDM symbols in an OFDM symbol dimension (time dimension) with respect to the same subcarrier.
As described above, the DVB-T standard and the ISDB-T standard insert spatially scattered SP signals in OFDM symbols, thereby reducing the redundancy of SP signals relative to primary information to be transmitted.
When transmission path characteristics are calculated using the SP signals, it is possible to identify the characteristics of the subcarriers having the SP signals inserted therein. However, it may be impossible to directly calculate the characteristics of other subcarriers including the primary information. Thus, a receiving side performs a filtering operation on SP signals using a two-dimensional interpolation filter, thereby estimating the transmission path characteristics of other subcarriers including the primary information.
Generally, the estimation of transmission path characteristics with the two-dimensional interpolation filter is performed in the manner described below.
To estimate the transmission path characteristics, information components are firstly removed from the received OFDM signals, and only SP signals inserted at the positions shown in FIG. 6 are extracted.
Then, using a reference SP signal, modulation components are removed from the extracted SP signals. The modulation-component-free SP signals show transmission path characteristics of subcarriers having the SP signals inserted therein.
Subsequently, the modulation-component-free SP signals are supplied to a time dimension interpolation filter, so that time dimension interpolation is performed and the transmission path characteristics of subcarriers having SP signals are estimated for each OFDM symbol. As a result, as shown in FIG. 7, it is possible to estimate the transmission path characteristics for every three subcarriers in the frequency domain of all OFDM symbols.
Then, as shown in FIG. 8, the time-dimension-interpolated SP signals are supplied to a frequency dimension interpolation filter, and threefold oversampling is performed on the signals, thereby performing frequency dimension interpolation. Accordingly, the transmission path characteristics of all of the subcarriers in the OFDM symbols are estimated. As a result, it is possible to estimate the transmission path characteristics for all of the subcarriers of the received OFDM signals.
Meanwhile, terrestrial broadcasting waves are transmission paths transmitted under a multipath environment. That is, terrestrial broadcasting waves are strongly affected by delay waves depending on the environments surrounding a reception position, such as geographical features and the arrangement of buildings. Signals received by OFDM receivers may be a combined wave of a direct wave and a plurality of delay waves.
Accordingly, under a multipath environment, a plurality of symbol boundaries exists since a plurality of paths exists. Generally, intersymbol interference is avoided by setting an FFT window on the basis of a symbol boundary position of an earliest arriving path.
Now, methods for setting a position of an FFT window that determines an FFT processing starting position will be described.
In a first method for setting an FFT window, an OFDM signal not having undergone an FFT operation is delayed so as to determine the correlation between a waveform of a guard interval and a waveform of a rear part of an OFDM symbol (i.e., a copy source signal wave of the guard interval), thereby determining a boundary of OFDM symbols (see, for example, Japanese Unexamined Patent Application Publication No. 2001-292125). In this method, a time point at which a value of an autocorrelation function becomes a maximum value indicates the boundary of the OFDM symbols of each path.
In addition, in a second method for setting an FFT window, the above-described SP signals are used. In this method, transmission path characteristics are estimated regarding all of the OFDM symbols by interpolating SP signals with a time dimension interpolation filter after extracting the SP signals from the OFDM signals and removing modulation components from the SP signals. Then, an IFFT operation is performed on the estimated transmission path characteristics so as to generate a delay profile representing the signal strength of each path, and the boundary of OFDM symbols is determined on the basis of the earliest arriving path.
In addition, a third method for setting an FFT window is also known. In this method, a waveform of a guard interval is extracted from an OFDM signal not having undergone an FFT operation, and consistency of this waveform and a waveform of a rear part of an OFDM symbol is determined, thereby determining a boundary of OFDM symbols. In this method, a delay profile representing the signal strength of each path by determining the consistency of the waveforms is generated, and the boundary of the OFDM symbols are determined on the basis of an earliest arriving path.
Furthermore, recently, a method in which the above-described second and third methods are combined has also been suggested (see, for example, Japanese Unexamined Patent Application Publication No. 2004-153831). In this method, it is possible to remove a false path by comparing the delay profiles generated according to two methods even if the false path caused by noise is included in the delay profiles.